The disclosure concerns a reactive ion etch process and reactor chamber for etching a thin film such as a dielectric film on a workpiece such as semiconductive wafer. Such processes can employ an etch process gas such as a fluorocarbon or fluoro-hydrocarbon gas whose plasma byproducts include etch species and polymer species. An plasma enhanced reactive ion etch process employing such a gas produces both etching of the thin film and deposition of polymer. We have discovered that uniformity of etch rate distribution and polymer deposition rate distribution near the edge of the workpiece is affected by the temperature of a process ring that surrounds the edge of the workpiece. Initially, immediately prior to plasma processing, the process ring is at the same temperature as a cathode on which the workpiece is held. A metal base layer underlying the cathode includes coolant flow passages for cooling the cathode. During plasma processing, the process ring temperature increases, so that successive workpieces are subject to different process conditions affecting etch rate distribution across the workpiece surface. In processing successive workpieces after igniting a plasma, the distribution of etch rate across the workpiece surface of the first workpiece will differ from that of subsequent workpieces, giving rise to non-uniformities between workpieces.
The process ring temperature increases above the temperature of the workpiece, affecting etch rate near the workpiece periphery. In addition, the higher temperature of the process ring and relatively lower temperature of the workpiece affects polymer deposition near the workpiece periphery. Such changes adversely affect uniformity of distribution of etch rate polymer deposition rate near the workpiece periphery.